1. Field of the Invention
This invention relates to a control system, and more particularly, to a control system required to have a good robustness to disturbances, such as a control system for a motor.
2. Description of the Prior Art
In conventional control systems, a PI controller (Proportional-Integral controller) has been widely used in order to improve the robustness to disturbances, or to suppress influences of disturbances. The PI controller can improve the robustness to disturbances more at lower frequency. However, the recent requirement to improve the robustness to disturbances has become more and more severe in some applications. For example, a control system for controlling the speed of a capstan motor in a video tape recorder requires a highly improved robustness to torque disturbances, because the size and the inertia of the capstan motor have been greatly reduced in recent years.
In order to achieve this requirement, one of the inventors of the present invention has proposed a control system with a highly improved robustness to disturbance in U.S. patent application Ser. No. 917498 filed Oct. 10, 1986. This control system comprises a detecting means producing a detected signal corresponding to a controlled variable of a controlled system, an error means producing an error signal corresponding to the detected signal of the detecting means, a memorizing means renewing a plurality of memorized values sequentially and periodically each by a value corresponding to a first mixed value which is obtained by mixing the error signal of the error means and at least a set of N memorized values of the plurality of memorized values, the set of N memorized values having been renewed at intervals of L renewing cycles, where each of N and L is an integer at least equal to 2 and an output means producing a control signal corresponding to a second mixed value which is obtained by mixing the error signal of the error means and one or more memorized values of the memorizing means, and supplying the controlled system with the control signal so as to control the controlled variable of the controlled system.
The control system has good improved robustness to disturbances, especially at the frequencies k*fe (k=0, 1, 2, . . . ), where EQU fe=1/(L.Tx)
Tx is a time of one renewing or sampling cycle period.
However we found that some more improvement is necessary for more severely controlling the speed of a capstan motor in a video tape recorder. More specifically we found that the speed of a capstan motor is greatly influenced by a fluctuation of tape tension due to torque disturbances from a take-up reel (or a supply reel). Furthermore the frequency of the fluctuation of tape tension changes according to the quantity of the tape wound on a take-up reel (or a supply reel).